This invention is drawn to the field of materials science, and more particularly, to a novel method for forming .gamma.-tetragonal boron.
Boron is a nonmetal and constitutes one of the elements of group IIIa of the periodic table. Among the growing commercial applications of the element is its use for case hardening various substrates such as beryllium when applied as a coating and as a strengthening agent in certain composite materials.
The known crystalline structures of boron include the .alpha.-rhombohedral, the .beta.-rhombohedral and the .gamma.-tetragonal forms. The .alpha.-rhombohedral phase of boron is known as the low-temperature form and can be produced by a variety of vapor-phase deposition techniques with substrate temperatures of from 800.degree. to 1000.degree. C. The .beta.-rhombohedral form of boron is commercially available in powder or solid form and is typically prepared by processing at temperatures in excess of 1500.degree. C. Reference in this connection may be had to an article entitled, "Structure and Polymorphism in Elemental Boron", incorporated herein by reference, by J. L. Hoard, appearing at pps. 42-52 in Advances in Chemical Series, No. 32, American Chemical Society, Washington, D.C., (1961).
It is known that .gamma. boron can be formed by chemical vapor deposition of boron on a substrate maintained at about 1100.degree.-1300.degree. C. The vapor source typically is either hydrogen reduction of boron trichloride (BCl.sub.3) or thermal decomposition of diborane (B.sub.2 H.sub.6). Reference in this connection may be had to an article entitled, "The Structure of Tetragonal Borons", by Hoard et al, appearing at Vol. 80 of the J. Am. Chem. Soc., pp. 4507-15, (1958), incorporated herein by reference. However, the deposition rate obtained by the known methods is usually a few thousandths of a millimeter per hour which requires long deposition times to produce comparatively thin deposits. In addition, the known techniques of .gamma. boron formation require relatively complex apparatus and the maintenance of high substrate temperatures.